It seems really better to have only one active and one passive participle, and then the endings are offered quite naturally by the existing languages, namely -nt and -t. The former is known from all the Romanic languages, from which a certain number are taken over into English: fluent, different, latent, defendant, etc. Compare also D -end (pronounced -ent): sehend, etc., and Sc -nde: seende, kallande, etc. The passive participle in -t is found in I S, in E in verbs like kept, in Sc regularly: set, kallat, in D in the weak verbs: gesegnet, gebracht, in R in some verbs.
Thus from the verbal stems ama love, protekte protect, mari marry, konstitu constitute, es be, we form the participles amant amat, protektent protektet, marient marit, konstituent konstitut, esent (no passive). It will be seen that while -nt can be added immediately after a, and -t after any vowel, the vowel -e- is put in between -i, -u or a consonant on the one hand, and -nt on the other. To these forms the endings -i, -e, -o, -a can of course be added: amanti fema, amanto lover, men amata by beloved one; marito husband, etc. Stress amánt(i), amát(i), etc.